1. Field of the Invention
The present invention relates to a light splitter and, more particularly, to a polarization light splitting film for use in a liquid crystal display device and the like.
2. Discussion of the Related Art
Due to recent technological development, liquid crystal displays (LCDs) are becoming increasingly popular for use in the display component of a personal computer and the like. Advantages of the liquid crystal display, such as the fact that they are thinner and lighter than the conventional display devices, for example, largely contributed to their gain in popularity. Moreover, a narrow viewing angle in conventional LCDs, which was previously considered to be a disadvantage, has been recently overcome by newly developed LCDs with a wide viewing angle. Accordingly, a wide variety of usages of the LCDs beyond personal-computer-use is expected to emerge.
Since a liquid crystal panel itself is not a light-emission device, the LCD needs an illumination light source. Reflection type LCDs use external illumination as the illumination light source. However, transmission type LCDs equipped with an internal backlight system are more popular.
FIG. 13 shows a typical structure of a conventional transmission type LCD. In this example, a backlight system 20 includes a light source 21, light guide 22, light diffusion elements 23, reflective sheet 24, diffusion sheet 25, and a prism sheet 26. Light emitted from light source 21 is incident on light guide 22. A cone-like shaped object indicates a viewing position of the LCD. The incident light propagates in the light guide 22 by experiencing multiple total internal reflections. A portion of the light in the light guide 22 is diffused (or scattered) upwards by light diffusion elements 23 and emerges from the light guide 22. A portion of the light emitted downwards from the light guide 22 is reflected by reflection sheet 24 and is returned to the light guide 22. The light emitted upwards from the light guide 22 is diffused by diffusion sheet 25 and converged by prism sheet 26. The resultant light is used as illumination light for a liquid crystal cell 30, which is sandwiched by polarizing plates 31 and 32.
In most LCDs, polarized light that is obtained by transmitting light through a polarizing plate is modulated in the liquid crystal layer. Approximately half of the incident light is absorbed at the polarizing plate and, accordingly, the efficiency in the light usage is low. In order to produce sufficient luminance, more light needs to be incident on the polarizing plate. However, increasing the light intensity causes a variety of problems, such as increased power consumption of the light source and adverse effects on the liquid crystal material due to heat generated from the thus powered up light source, which degrades the display quality.
To solve the above-mentioned problems, the following technique has been proposed. First, unpolarized light from a light source is split by a polarization light splitter into two linearly polarized light beams having the polarization directions orthogonal to each other. Then, one of the linearly polarized light beams is directly used for illumination while the other polarized light beam is used indirectly. In other words, one of the polarization components, which are separated by the polarizing plate, is directly incident on the liquid crystal cell, whereas the other polarization component, which progresses towards the light source, is re-directed toward the polarizing plate by reflection or the like. This way, the efficiency in light usage can be improved. Some of the recent developments along this direction are as follows.                (1) In Japanese Application Laid-Open Publication No. 04-184429, an unpolarized light beam from a light source is split by a polarization light splitter into two orthogonally polarized light beams. One of the polarized light beams is directed towards the liquid crystal cell. The other beam, which progresses towards the light source, is converged and then reflected towards the liquid crystal cell.        (2) A backlight system disclosed in Japanese Application Laid-Open Publication No. 06-265892 includes a beam deflector that transmits light in a direction substantially normal to the light emitting surface of a planar light guide. The deflector is disposed on the light emitting side of the planar light guide, and a polarization light splitter is disposed above the deflector.        (3) In a backlight system disclosed in Japanese Application Laid-Open Publication No. 07-261122, a polarized light splitter is located on the light emitting side of a parallel light generating device. The parallel light generating device is constructed of a light scattering conductor including a portion having a wedge-shaped profile.        (4) Similar systems are proposed in Japanese Application Laid-Open Publication No. 06-289226, No. 07-49496. All the proposed polarized light splitting systems, including the above-mentioned (1) to (3), employ a multi-layer film utilizing the Brewster law (Brewster's angle) as polarization splitting means.        
FIGS. 11 and 12 show cross-sectional views of conventional polarization light splitting films. In FIG. 11, polarization light splitting film 40 has a multi-layer structure formed by alternately laminating light transmission layers 41 having a large refractive index and light transmission layers 42 having a small refractive index. Using Brewster's law, the polarization light splitting film 40 is designed to transmit the p-polarized light component and reflect the s-polarized light component.
Polarization light splitting film 40 in FIG. 12 also has a laminated structure of two types of layers 43, 44 having different refractive indices. The thickness of the layers 43 is designed to cause interference with respect to visible light. In this construction, if the refractive indices of the layers 43, 44 and the thickness of layer 43 satisfy a certain predetermined relationship, the transmission contrast between the p-polarized component and the s-polarized component becomes large with respect to a certain incident angle. The polarization light splitting film 40 of FIG. 12 utilizes this property to transmit polarized light.
However, the above-mentioned conventional techniques have the following disadvantages. The backlight system of (1) is intended for use with projection LCD devices. The structure of such an illumination system requires a large amount of space. Therefore, it cannot be applied to thin panel-type LCD devices.
The backlight system of (2) is applicable to thin LCD devices. If a polarized light splitting layer is fabricated on the inclined sides of a columnar prism array having a plurality of triangular-shape prism units, a relatively high efficiency can be achieved in light usage. However, the structure of the polarized light splitting layer becomes complicated. In particular, it is difficult to fabricate the polarized light splitter layer on the inclined sides of the columnar prism array. This type of backlight system is therefore not suitable for mass production.
As for the backlight system of (3), if the parallel light generating device is constructed of a certain light scattering conductor having a wedge-shaped profile, superior efficiency in light usage can be obtained. However, it is difficult to manufacture such a light scattering conductor with a desirable light scattering performance. Accordingly, this system is not suitable for actual use.
In addition, since all the systems discussed in (1) to (4) utilize the Brewster law (or Brewster's angle), it is necessary to form multiple layers on an inclined face. Therefore, these systems require a complicated structure and are not suitable for mass production.
Furthermore, to produce a sufficient polarization light splitting property using the conventional multi-layer structures above, at least five layers need to be laminated, resulting in a complicated layer structure. In addition, the polarization light splitting property depends on the incident angle and the wavelength, etc., imposing many undesirable limitations in the actual use. For example, because illumination light that propagates in a direction normal to the emitting surface of the backlight system is polarized and split using Brewster's law, the multiple layers need to be inclined with respect to the emitting surface by forming the layers on an inclined plane. Therefore, the polarization light splitting device must be thick enough to accommodate this structure. This is not consistent with a recent trend towards reducing the thickness of LCD devices.
In addition, the conventional polarization light splitting devices would not be effective unless the coupling of the light splitting device with other elements of the backlight system, such as a diffusion film and a prism sheet, is optimized. Also, it is important to add anti-reflection layers, hard coating layers, adhesive compound/cement layers, and/or polarizing layers to the polarized light splitting device for the whole device to be effective under various operating conditions. However, there have been no such proposals.
As described above, some of the conventional backlight systems use a laminated structure of a plurality of layers having different refractive indices. Other systems use the multi-layered structure formed on a prism-like shaped object. In any event, the conventional backlight systems are limited in the sense that they use the Brewster law (Brewster's law).